1. Field of the Invention
The present invention relates to an air conditioning control apparatus for vehicles which is provided with PTC (Positive Temperature Coefficient) heaters for boost heating as well as a heater core that uses engine cooling water for its heat source.
2. Description of the Related Art
The prior art for this type of air conditioning control apparatus for vehicles includes, for instance, the type disclosed in Japanese Unexamined Patent Publication No. H 5-169967. In this apparatus, an evaporator and a heater core for controlling air temperature are provided inside an air conditioning duct system. Outlets (defrost outlet, ventilation outlet, foot outlet) that are opened/closed by mode doors are formed at the downstream-most side of the air conditioning duct system. A PTC heater further provided at the foot outlet so that power is supplied to the PTC heater until the temperature of the engine cooling water having rises to a high level so that the heating by the heater core becomes sufficient to assure warm air through the foot outlet.
The structure in the prior art described above is effective for obtaining a comfortable feeling of warmth for passengers since discharge of warm air at the foot level is given priority even during the initial stage of heating, in which the temperature of the engine cooling water is low. However, the structure cannot assure effective defrost during the initial stage of heating, since the PTC heater is provided at the foot outlet only.
In other words, unless the FOOT mode is not selected as the discharge mode during the initial stage of heating (i.e., if the DEFROST mode, which gives priority to removing frost and fog, or the DEF/FOOT mode for simultaneously achieving removal of frost and fog on the windshield and a comfortable feeling of warmth for the passengers is selected) an insufficient quantity of warm air is supplied through the defrost outlet in the structure described above, since priority is still given to discharging warm air at the foot outlet. Consequently, there is no choice but to wait for the temperature of the engine cooling water to increase.
On the other hand, if the quantity of heat supplied through the defrost outlet is set equal to the quantity of heat supplied through the foot outlet, it will become difficult to achieve the feeling of comfort and warmth afforded when the feet are warmed and the head is not subjected to a direct flow of hot air. Thus, if one must choose, it is preferable to put more emphasis on the discharge of warm air at the foot outlet.
In addition, a PTC heater, which utilizes a battery or the like for power, does not contribute to the heating of discharge air if the DEFROST mode is selected during the initial stage of heating. This is because the foot outlet is closed, so no air will then flow around the PTC heater provided at the foot outlet, resulting in a wasteful consumption of power.